1. Field of the Invention
The present invention relates to a wavelength division multiplexing system using a multimode optical fiber. More particularly, the present invention relates to a wavelength division multiplexing system using a multimode optical fiber that exhibits a high transmission bandwidth in a wide wavelength range, which realizes a wavelength division multiplexing system using a multimode optical fiber which has not been possible previously.
Priority is claimed from Japanese Patent Application No. 2004-13075, filed Jan. 21, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
Multimode optical fibers (MMFs), such as GI (graded index) fibers, which have a large core radius and a high numerical aperture (NA), have been widely used as transmission paths in optical local area networks (LANs). Driven by a need for a faster LAN, profiles of conventional GI fibers are controlled very precisely, and further improvement in performance seems almost impossible at present. In order to increase a transmission bandwidth (hereinafter referred to as “bandwidth”) of multimode optical fibers having a wider transmission bandwidth than those currently available, the use of wavelength division multiplexing (WDM) has been desired.
However, an optimal profile of GI fibers is dependent on wavelength, and GI fibers that are optimized for a certain wavelength are not suited for wavelength division multiplexing since such fibers exhibit a very limited bandwidth at wavelengths other than the optimal wavelength. FIG. 1 shows wavelength characteristics of 50/125 μm fibers, each optimized for a wavelength λ0 of 850 nm or 1300 nm, and having a maximum relative refractive index difference Δ of 0.01 and a core radius “a” of 25 μm in the overfilled-launch (OFL) bandwidth (see IEC 60793-1-49 Specification). As shown in the graph, the bandwidths drop sharply as wavelengths departs from the optimal wavelengths.
For calculation in the example shown in this graph and other examples, values of material dispersion indices of pure silica and germanium-doped silica discussed in N. Shibata and T. Edahiro, “Refractive-index dispersion for GeO2—, P2O5— and B2O3-doped silica glasses in optical fibers,” Trans. IECE Japan, vol. E65, pp. 166-172, 1982, were used, and the value of a material dispersion index of fluorine-doped silica discussed in J. W. Fleming, “Material Dispersion in Lightguide Glasses,” Electron Lett., vol. 14, pp. 326-328, 1978 was used, and RMS spectrum range of incident light was assumed to be 0.35 nm. Bandwidths were calculated based on group delays of each mode calculated from the profiles (see K. Okamoto, “Comparison of Calculated and Measured Impulse Responses of Optical Fibers,” Appl. Opt., vol. 18, pp. 2199-2206, 1979).